1. Field of the Invention
The present invention relates generally to an array substrate for a display device, and more particularly to the structure of an undercoat layer between an insulating substrate and a semiconductor layer.
2. Description of the Related Art
An array substrate, which is applied to an active-matrix flat-panel display device such as a liquid crystal display device or an organic electroluminescence display device, includes switching elements which are disposed in matrix-arrayed pixels. For example, in a case where a top-gate type thin-film transistor, which includes a semiconductor layer formed of amorphous silicon or polysilicon, is applied to the switching element, an undercoat layer is disposed between an insulating substrate and the semiconductor layer.
As is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-163353, for instance, the undercoat layer, in usual cases, adopts a double-layer structure in which a silicon nitride (Si3N4) film on the insulating substrate and a silicon oxide (SiO2) film on the silicon nitride film are stacked. The semiconductor layer is disposed on the silicon oxide film.
In the process of fabricating the array substrate, it is possible that dust adheres to the insulating substrate or particles may mix in a step of forming the undercoat layer and semiconductor layer. In the case where the undercoat layer of the above-described double-layer structure is adopted, it is possible that a cleaning agent may permeate in foreign matter such as dust or particles in a cleaning process step which follows the formation of the semiconductor layer. In particular, in the case where hydrogen fluoride (HF) is used as a cleaning agent, foreign matter is dissolved and a void occurs. In worst cases, even the insulating substrate is etched, leading to deterioration in manufacturing yield.
The layer undercoat layer of the double-layer structure is formed of, e.g. a silicon nitride film with a thickness of 50 nm and a silicon oxide film with a thickness of 100 nm. In the case where a semiconductor layer with a thickness of about 50 nm is disposed on the undercoat layer, if foreign matter of a sub-micron level is present, it is difficult to completely cover the foreign matter with these films. In addition, if the thicknesses of the silicon nitride film and silicon oxide film are simply increased, deterioration may occur in optical characteristics of the display device, such as transmittance and chromaticity. Thus, the thicknesses of these films are set in a limited range.